Internal combustion engines provided with additional air to the scavenging ducts are known. They reduce fuel consumption and exhaust emissions, but it is difficult to control the air and fuel ratio in these types of engines. Further, it can be difficult to substantially reduce the exhaust emissions utilizing these types of engine designs because uncombusted fuel may flow through the engine and out through the exhaust system as a pollutant.
In a recently published SAE-report with reference No. 2000-01-900, an engine is described of the two-stroke type. By way of check-valves, so called reed-valves, the two scavenging ducts located closest to the exhaust port are fed with so much air that it is sufficient for the whole scavenging process. One or several more scavenging ducts with ports located close to the inlet side will instead scavenge air and fuel-mixture at the same time as the other ports will scavenge air. It is pointed out that this scavenging takes place in parallel; that is, it begins at the same time and continues throughout the entire scavenging process. The principle is described as being stratified scavenging in space. Compared to a conventional two-stroke engine, the fuel consumption and exhaust emissions will be reduced. At the same time, however, it is noted that at least some of the air and fuel-mixture will be lost through the exhaust gas port at the end of the scavenging process, that is during the last 40 to 50 crank angle degrees before the exhaust gas port is closed. Obviously this loss is undesirable. Furthermore, check valves are used for feeding the scavenging ducts located close to the exhaust gas port in a known way. The flow restriction at the check valves complicates the filling of these ducts with air. These types of check valves, usually called reed-valves, however, have a number of other disadvantages. They often have a tendency to come into resonant oscillation, and can have difficulties to cope with the high rotational speeds that many two-stroke engines can reach. Besides, inclusion of the valves results in added cost and an increased number of engine components.
International Patent Application WO98/57053 shows a few different embodiments of an engine where air is supplied to the scavenging ducts via L-shaped or T-shaped recesses in the piston. Check valves are thus missing. Air is supplied to all the scavenging ducts and serves as a buffer against the subjacent air and fuel-mixture. The scavenging is thus stratified in time, but not in space in contrast to the engine mentioned above. In all embodiments, the piston recess has, where it meets the respective scavenging duct, a very limited height, which is essentially equal to the height of the actual scavenging duct. A consequence of these designs is that the passage for air delivery through the piston to the scavenging port is opened considerably later than the passage for air and fuel-mixture to the crankcase by the piston. The period for the air supply is thus significantly shorter than the period for the supply of air and fuel-mixture, where the period can be counted as crank angle or time. This can complicate the control of the total air and fuel ratio of the engine. This also means that the amount of air that can be added to each scavenging duct is significantly reduced because the underpressure driving this addition of air has decreased considerably since the inlet port has already been open during a certain period of time when the air supply has been opened. This implies that the period and the driving force for the air supply are both small. Furthermore, the flow resistance in the L-shaped and T-shaped ducts is relatively high, partly because the cross-section of the duct is small close to the scavenging port and partly because of the sharp bends created by both the L-shape or T-shape. When the air has just passed into the scavenging port, it is forced to change direction abruptly away from the lateral direction of the cylinder to instead follow the scavenging duct outwards and then downwards, i.e. two curves, each of 90 degrees and in rapid succession. This is due to the fact that the scavenging ducts of the engine are running in a radial direction to the cylinder. Each of these features contribute to increasing the flow resistance and to reducing the amount of air that can be added to the scavenging ducts, and in turn decreases the possibilities to reduce fuel consumption and exhaust emissions by means of this arrangement.
The present invention refers to a crankcase scavenged internal combustion engine of two-stroke type having at least one cylinder and at least one air passage arranged between an air inlet and the upper part of at least two scavenging ducts with scavenging ports located close to the exhaust port of the cylinder. At least one intake orientated scavenging port is located close to the inlet port of the cylinder and is fed by at least one scavenging duct or similar structure. The air passage and the scavenging ducts are so arranged that the scavenging ducts can be supplied with, and hold so much air that during the following scavenging process they will scavenge essentially nothing but air. Fresh air is thus added into the scavenging ducts located closest to the exhaust gas port and this fresh air is intended to serve as a buffer against the exhaust gas port for the air and fuel-mixture that is supplied more closely to the inlet port. It should also be pointed out that by this configuration and function, fuel consumption and exhaust gas emissions are reduced. Additionally, engines of the type disclosed herein are particularly suitable for powering handheld working tools because of their compact and lightweight nature.
In at least one embodiment, the presently disclosed invention, takes the form of an internal combustion engine characterized in that an air passage is arranged from an air inlet that may be provided with a restriction valve and that is controlled by at least one engine parameter such as the carburetor throttle control. The intake orientated scavenging port/s is/are arranged so that it/they begin to scavenge air and fuel-mixture later than the scavenging ports located adjacent to the exhaust outlet begin to scavenge air.
Since the intake oriented scavenging ports begin to scavenge the air and fuel-mixture later than the exhaust orientated scavenging ports begin to scavenge air, the air and fuel-mixture will have shorter time to reach the exhaust port. In this way the losses of the air and fuel-mixture through the exhaust port can be reduced. This is primarily achieved by at least partly filing the intake orientated scavenging ports with air or exhaust gases before the scavenging process begins. In this way, the added scavenging air will be scavenged first, which will delay the scavenging of the air and fuel-mixture. Furthermore, the air and fuel-mixture intake orientated scavenging ports can also be arranged so that their respective upper edge will be located axially lower than the corresponding edge of the other scavenging ports. This also delays the scavenging of the air and fuel mixture, but based on the action of the piston and its cooperation with the scavenging ducts.
Because at least one connecting port in the engine""s cylinder wall is arranged so that it, in connection with piston positions at the top dead center, is connected with flow paths arranged in the piston, the supply of fresh air to the upper part of the scavenging ducts can be arranged entirely without check valves. This is possible because at positions at or near top dead center, there is an underpressure in the scavenging duct in comparison to the ambient air. Consequently, a piston ported air passage without any check valves can be arranged; and this is a significant advantage. Since the air supply has a very long period of time, a substantial amount of air can be added, so that a very satisfactory exhaust emission reduction rate can be achieved. Control is applied by means of a restriction valve in the air inlet, preferably controlled according to at least one engine parameter. Such a control strategy and design is a considerably less complicated solution than a variable inlet. The air inlet has preferably two connecting ports, which in one embodiment are so located that the piston covers them when in the bottom dead center position.
The restriction valve can preferably be controlled by the engine""s throttling or rotational speed, alone or in combination with other engine parameters. These and other characteristic features and advantages will become more apparent from the following detailed description of various embodiments, supported by the included drawing figures.